1. Field of the Invention
The present invention relates to a process for the preparation of ferrite magnetic particulate for magnetic reocrding, and more particularly relates to a finely divided ferrite magnetic material, which is suitable for use in perpendicular magnetic recording system.
2. Description of Prior Arts
Heretofore, there has been used a magnetic recording system wherein a recording medium such as magnetic recording tape is magnetized along its longitudinal direction in the surface area thereof. A perpendicular magnetic recording system has been recently proposed for accomplishing high density magnetic recording, and various magnetic mediums for use in this system are under study.
There are known methods wherein a magnetic material layer is formed on a substrate (i.e., support) such as a film by a sputtering process, a vacuum deposition process, etc. as the method of preparing the magnetic recording medium for use in the perpendicular magnetic recording system. For example, there has been developed a magnetic recording medium wherein a layer of a magnetic material such as cobalt-chromium is formed on a substrate by the sputtering process.
However, the process for preparing the magnetic recording medium utilizing said sputtering process or vacuum deposition process has disadvantages in productivity and product quality as compared with conventional process for preparing the magnetic recording medium utilizing the conventional coating process. Accordinagly, methods utilizing the coating process are also studied as the process for preparing the magnetic recording medium for use in the perpendicular magnetic recording system.
For example, there has been proposed a process for preparing the magnetic recording medium for use in the perpendicular magnetic recording system wherein a hexagonal ferrite such as hexagonal barium ferrite in the form of fine particles having a hexagonal plate shape is used as a magnetic material, the hexagonal ferrite is mixed with and dispersed in a resin (binder) and the surface of a support is coated with the ferrite.
As typical processes for preparing the hexagonal ferrite such as hexagonal barium ferrite used as the magnetic particulate for the above-mentioned magnetic recording medium, there are known wet processes (such as co-precipitation process and hydrothermal synthetic process), vitrification process, etc.
The present invention provides an improved process for the preparation of hexagonal ferrite magnetic particulate by the vitrification process.
The preparation of the hexagonal ferrite magnetic material by the vitrification process is generally carried out in the following manner. A mixture of starting materials containing the desired ferrite component and a glass-forming component is melted and then rapidly cooled to form an amorphous material which is then heat-treated as such to form and deposit hexagonal ferrite crystals therefrom, and other materials such as glass component, etc. than the ferrite crystals are removed from the material obtained from the above heat treatment (hereinafter referred to as heat-treated material).
The starting material mixture for used in the preparation of the hexagonal ferrite magnetic material by the vitrification process usually contains a basic component for the hexagonal ferrite, a coercive force-reducing component and a glass-forming component.
As the basic component for the hexagonal ferrite, Fe.sub.2 O.sub.3 in combination with a metal oxide or oxides such as BaO, SrO and PbO is used. As the coercive force-reducing component, a combination of a divalent metal oxide or oxides such as CoO, NiO and ZnO with a tetravelent metal oxide or oxides such as TiO.sub.2, ZrO.sub.2 and HfO.sub.2 is used. As the glass-forming component, boron oxide (B.sub.2 O.sub.3) is generally used. Alternatively, there may be incorporated in the starting material mixture the above-mentioned components in the form of compounds or salts (for example, carbonate, nitrate and boric acid for boron) capable of being converted into the above-mentioned oxides under heating conditions in the melting step of the starting material mixture.
The ferrite prepared from the above-mentioned starting material mixture is a magnetoplumbite type hexagonal ferrite having the following formula: EQU RFe.sub.12-2x M.sub.x M'.sub.x O.sub.19
wherein R is at least one metal atom selected from the group consisting of barium, strontium and lead, M is at least one divalent metal atom selected from the group consisting of cobalt, nickel and zinc, M' is at least one tetravalent metal atom selected form the group consisting of titanium, zirconium and hafnium, and x is a number ranging from 0.6 to 1.0.
The ferrite magnetic material in the shape of hexagonal plate, which is suitable for use in the perpendicular magnetic recording system, is in the form of fine particles wherein the diameter of the hexagonal plate is 0.1 .mu.m or below and the thickness thereof is 0.03 .mu.m or below. In order to prepare such magnetic material in the form of fine particles, there is used in the conventional vitrification process, for example, a process wherein the molten starting material mixture containing the above-mentioned components is quenched to convert it into an amorphous material which is subsequently heat-treated to form the deposit hexagonal ferrite crystals. As the method of quenching the molten mixture, there is used a method wherein the molten starting material mixture is brought into contact with the surface of rotating metallic rolls by pouring the molten mixture onto said surface (called roll method).
As described hereinbefore, the magnetic recording medium for use in the perpendicular magnetic recording system utilizing hexagonal ferrite magnetic particulate is produced by forming on a substrate a magnetic particulate-containing layer in which said magnetic particulate is dispersed in a binder. It is desirable for the magnetic recording medium that the noise possibly generated therein in the course of the magnetic recording stage and regenerative output stage is suppressed. The intensity of noise generation in the magnetic recording medium is related to the size of ferrite magnetic particulate used therein. In general, the smaller the size of the magnetic particulate used therein becomes, the lower the noise of the magnetic recording medium generated during the magnetic recording or regenerative output stage becomes. This fact is valid for the magnetic recording medium prepared by the coating method, using the hexagonal ferrite magnetic particulate. Accordingly, from the viewpoint of the reduction of noise during magnetic recording or regenerative output stage, it is desirable that hexagonal ferrite magnetic particulate has a particulate size of as small as possible.
In the preparation of the hexagonal ferrite magnetic particulate by the above-mentioned conventional vitrification process, it is possible to reduce the size of the resulting ferrite magnetic particulate by employing a relatively low temperature for the heat treatment in the process of the production and deposition of the hexagonal ferrite crystals in the amorphous material. However, if the size of ferrite magnetic particulate is reduced by performing the heat-treatment at a low temperature, the magnetic characteristics, in particular the saturation magnetization, seriously decreases. That is, in the conventional vitirificatin process, the lower the heat-treatment temperature on the amorphous material becomes, the smaller the size of the resulting ferrite magnetic particulate becomes, and accordingly the lower the saturation magnetization of ferrite magnetic particulate becomes. The extent of reduction of the saturation magnetization owing to the reduction in the particulate size is surprisingly great. Therefore, there is a great need for the process for preparing the ferrite magnetic particulate with higher saturation magnetization than that obtained by the conventional vitrification process, when compared at the same particulate size level. In other words, a process for the preparation of the hexagonal ferrite magnetic particulate which gives a ferrite magnetic particulate with smaller size than that obtained by the conventional vitrification on the equal saturation magnetization basis is desired.